Solid amorphous silicon-boron-hydrogen alloy films have heretofore been utilized as electronic materials; see, "Fundamentals of Solar Cells", chapter 11 section 2, Alan L. Fahrenbruch and Richard H. Bube, Academic Press 1983. Most commonly they are used in the form of deposits, prepared by radio frequency glow discharge decomposition of hydrides, in solid state devices converting solar radiation to electrical energy. Preparation and electrical current-voltage characteristics of semiconductor junctions made of thin films of amorphous silicon doped with trace amounts of boron are described by W. E. Spear, P. G. Le Comber, S. Kinmond and M. H. Brodsky, "Amorphous Silicon p-n Junction", Applied Physics Letters, Vol. 28, No. 2, pg. 105, Jan. 1976. In this work, the fact that materials prepared by decomposing the hydride gases always contain hydrogen, and the role that this hydrogen plays in forming the amorphous structure advantageous for electronic applications, were not considered. Hydrogen content and bonding configurations and their influence on growth and structure of amorphous silicon films prepared by decomposition of silane have been examined by J. C. Knights, "Growth Morphology and Defects in Plasma-Deposited a-SI: H Films", Journal of Non-Crystalline Solids, Vol. 35 & 36 (1980), pages 159-170.
The high efficiency of solar cells incorporating thin films of hydrogen-containing amorphous silicon-boron has motivated scientific interest in the solid state physical properties of compositions exceeding in boron content the low concentrations utilized for semiconductors. For example, C. C. Tsai in an article entitled "Characterization of Amorphous Semiconducting Silicon-Boron Alloys Prepared by Plasma Decomposition" published in Physical Reviews, Volume 19, page 2041, February 1979, describes structure, optical absorption, electrical conductivity and paramagnetic resonance of hydrogenated boron-silicon alloys, ranging in composition from 0 to 100% boron. In this work, films deposited by glow discharge decomposition of Si H.sub.4 +B.sub.2 H.sub.6 gas mixtures on a variety of substrates, including glass, aluminum, crystalline silicon, and saphire have been studied. The relative efficiency of incorporation of boron from the gas into the film, compared to that of the silicon, is about 0.65 + or - 0.15 at 270.degree. C. deposition temperature. Therefore, amorphous hydrogenated silicon-boron alloys with any composition in the binary system can be made by properly choosing the Si H.sub.4 -B.sub.2 H.sub. 6 gas mixture. The hydrogen content in the films has been found to range between 10 and 45 atomic percent depending on the deposition parameters. In the film structure, hydrogen forms B-H and Si-H bonds which represent the majority of the amorphous atomic lattice. The structure and stability of the films depend in great measure on the deposition temperature. Films deposited at 270.degree. C. are dense, contain less than 1 atom percent of oxygen and are very stable to exposure to ambient air. They loose their hydrogen when heated to about 350.degree. C. to 550.degree. C. In contrast, films deposited at 25.degree. C. are porous, easily oxidize upon exposure to air, and start loosing their hydrogen near 350.degree. C.
In the current electronic art, also pyrolysis, or thermal decomposition, of gaseous hydrides is practiced for preparation of amorphous hydrogen-containing silicon and boron films. An example is the work of B. G. Bagley, D. E. Aspnes, A. C. Adams and R. E. Benenson described in the paper entitled "Optical Properties of LPCVD aB(H)" published in the Journal of Non-Crystalline Solids, Vol. 35 & 36 (1980), page 441. The paper reports the infra red and near ultra violet absorptions of films deposited on single crystal silicon substrates by pyrolysis of diborane at temperatures between 290.degree. C. and 400.degree. C. The films have remained stable upon exposure to laboratory atmosphere for six months without any degradation by oxidation.
Applicants are unaware of any other uses set forth in the prior art for silicon-boron-hydrogen alloys. Such alloys have been studied, developed and used exclusively for the purpose of utilizing their physical behavior as electronic semiconductors. The only chemical aspects of these materials that are recognized by the electronic art are the reactions occurring in the process of their fabrication as solid thin films, by decomposition of gaseous silicon and boron hydrides (silanes and boranes). Accordingly, the prior art teaches the composition and pressure of the gases from which the films are formed and the resulting film compositions, temperature and rate of their nucleation and growth, atomic lattice structure and stability to temperature and exposure to ambient air. The art also implies that the silicon-boron-hydrogen films can be deposited on almost any and all known solid substrates including metals, ceramics and organic resin materials. However, the art is devoid of any teaching or consideration of the chemical behavior of the films towards the substrate on which they have been deposited and towards materials which come in contact with them from the outside.
In the nonanalogous art of joining a specific material to itself or to other materials, a structural transition interface is utilized between the materials. This transition interface is created on atomic and crystal lattice levels by direct chemical reactions between the two surfaces in processes such as diffusion or fusion welding or bonding. In many instances, for convenience of lower bonding process temperatures, an intermediate filler material is interposed between the two surfaces to interact with each one of them separately and thus bond them together. Examples of this kind of state of the art processes are arc welding, brazing, soldering, and organic adhesive bonding. All these chemically generated bond interfaces create a region of transition in which the interfaced composition and crystalline structures are forced to adjust to each other. Particularly in the cases of soldering, brazing and adhesive bonding the filler material is retained at the joint and constitutes an additional adventitious material in the joined assembly. In general, the chemical and physical characteristics of the transition region are quite different from those of the bonded materials and pose problems and disadvantages of diminished mechanical strength, sensitivity to thermal or mechanical shock, or to chemical attack, undesirable electrical or thermal conductivity, and the like.
Advantages of using boron and/or silicon as active reactants in creating bonds between metals have been recognized by the current art, as shown, for example, in U.S. Pat. Nos. 2,714,760; 2,868,639; 3,188,203; 3,530,568 and 3,678,570. These patents teach joining high temperature corrosion and oxidation resistant iron, nickel and cobalt base alloys by means of brazing compositions based on nickel-chromium, or nickel-cobalt-chromium, to which boron, or boron and silicon, are added. Boron and/or silicon lower the melting point of the brazes and, thus, the joint can be made at temperatures low enough to avoid deteriorating the mechanical properties of the bonded alloys. Further advantage of using boron and silicon in the brazes is that although they act to generate a brazing liquid at a conveniently low temperature, they also at the same time tend to diffuse out of the bond into the metal and cause the liquid braze bond to solidify. In this way, the remelt temperature of the braze is raised above that of the original braze composition and the bonded assembly can be put to use at desirably high temperatures. However, the use of boron and silicon in this manner does not obviate the aforesaid difficulties associated with prior art bonding techniques.